In many applications in control systems for plants, including nuclear power plants, it is desirable or even necessary to sense the status of switch contacts. The switches may be main control board push buttons or selector switches, or they may be limit switches located on motordriven actuators. In either case, it is crucial that the process control system be able to sense the switch status reliably. One measure used to enhance the reliability of such systems involves the use of paired or redundant contacts, in which a switch has at least two pairs of contacts which are always in opposite states. A switch with form "C" contacts or form "D" contacts is such a switch. In the form "C" switch, there are two pairs of switches, one of which is normally open and the other of which is normally closed. Opening one switch automatically closes the other, and vice versa. In a form "C" switch, the contacts are arranged in a "break before make" fashion; in a form "D" switch, they are arranged in a "make before break" fashion. In either case, the switch may be inferred to be malfunctioning if both pairs of contacts have the same state after a short transition time, that is, both are open or both are closed. Therefore, to test the switch reliably, it is necessary only to provide circuitry which is capable of monitoring the condition of each half of the switch, i.e., each pair of contacts, and which produces an error signal if both pairs of contacts are in the same state. In conventional circuitry, this is accomplished by providing a digital indication of the status of the pair of contacts, e.g., a "0" if the pair of contacts is opened, and a "1" is the pair of contacts is closed. The signal indicative of the state of one pair of contacts is then compared logically with that of the other pair of contacts. If the signals have the same logical value (if the parity of a signal which a combination of the two is even) the switch is detected as malfunctioning, and appropriate corrective measures may be instituted.
While such circuitry is sufficient to insure reliable operation of an overall system in many instances, it is often necessary to provide an even higher level of reliability of periodically testing the testing circuitry itself. In the past, this higher-level testing has been simply a maintenance procedure carried out on the order of once every six months or so. Again, while this periodic testing is sufficient to insure a sufficient degree of reliably in many systems, there are advantages to be achieved if testing made automatic so that it can easily be carried out more frequently and even on-line.